As known, data is written (programmed) to a flash memory by means of hot-electron injection into floating-gate electrodes (hereinafter, floating gates) of the memory cells. To erase the data, electrons are extracted from the floating gates of the memory cells by means of the mechanism known as Fowler-Nordheim tunnelling at high electric fields, giving rise to a Fowler-Nordheim current.
In conventional flash memories, the erase operation has a global character, affecting the whole array (also called matrix) of memory cells or, where memory sectors are provided with, a whole memory sector. All the memory cells of the matrix or memory sector to be erased are submitted to an erase voltageVGB=VG−VBwhere VG is, for example, a negative voltage (e.g., ranging from −8V to −9V) applied simultaneously to all the word lines of the matrix or memory sector, that is, to the memory cells' control-gate electrode (hereinafter, control gate), and VB is a positive voltage (e.g., ranging from 8V to 9V) applied to the common substrate or bulk electrode of the memory cells of the matrix or memory sector, that is, the (typically P-type) doped semiconductor well in which the memory cells of the matrix or memory sector are formed. Starting from a prescribed initial value, the erase voltage VGB is progressively increased (in absolute value) until, by progressive extraction of electrons from the memory cells' floating gates, the threshold voltage of all the memory cells is brought below a prescribed value, chosen to assure a proper margin compared to the standard memory read conditions.
The global character of the erase operation is a significant disadvantage of conventional flash memories. In fact, even if memory sectors have the minimum memory-sector size that can be practically achieved at a reasonable cost in terms of semiconductor chip area. This minimum size is still some number of KBytes. This means that when even a single data byte or word belonging to a given memory sector is to be modified, the whole memory sector, that is, some number of KBytes of memory space, must be erased and then rewritten.
This limits the otherwise highly desirable use of flash memories in those applications that require frequent modification of single data bytes or words.
As a solution to this problem, flash memories have been proposed in which, during an erase operation, only one word line (more generally, only a subset of the set of word lines making up the memory matrix or sector) is biased to a negative voltage VselG of, e.g., −8 V to −9 V, while the remaining word lines of the matrix or memory sector are biased to ground. In this way, only the memory cells belonging to the selected word line(s) are subjected to the erase voltage VGB. In the memory cells belonging to the non-selected word lines, the electric field across the gate oxide thereof is substantially reduced so as not to trigger the Fowler-Nordheim tunneling.
Thanks to the above solution, which requires a suitable modification of the conventional row address decoder and row selection circuits, the flash memory can be erased with a finer granularity, equal to one word line (or a subset of word lines) instead of the whole matrix or sector. Defining a “memory page” as the elementary memory unit that can be individually erased, that is, one word line (or a subset of word lines), the memory can be referred to as a “Page Erasable ROM” or “PEROM”.
A problem affecting the PEROM is that when a memory page is to be erased, the memory cells not belonging to that memory page, but belonging to the same memory sector, (or to the memory matrix, if no memory sectors are provided) are disturbed. In fact, even if the gate-bulk voltage −VB, to which such memory cells are subjected, is not sufficient to erase them, such a voltage is, however, still favorable to the extraction of electrons from the floating gates thereof, and, thus, to a small reduction of the their threshold voltages. Considering that any given memory page can be erased and rewritten many thousands of times, some memory cells, even if not belonging to the word line(s) to be erased, may, at a given time, loose the datum stored therein.